The invention pertains to the art of printheads and, more particularly, to a method of controlling piezo elements in a printhead of a droplet generator.
An example of a method for operation of a printhead of an ink jet printer is disclosed in WO 95/25011. That printhead has a multitude of adjacently arranged channels, each of which is allocated to a nozzle. By activation of a channel, a droplet of ink is expelled from the respective nozzle. Through impulse control, the result is obtained that pressure waves within an activated channel will fade more rapidly. With this solution, the amplitude values of the impulses are adjusted, for which purpose linear amplifiers are needed. Such devices, though, have a poor efficiency and require expensive electronic components. The pulse widths are limited to whole number multiples of an acoustic period L/c, wherein L represents the length of the channel and c the sound velocity in the liquid. It is only possible, due to the complexity of the impulses, to operate all channels with the same impulse voltage and with the same pulse width.
Another example of an operating process for a piezo-electrical printhead is shown in U.S. Pat. No. 5,461,403. The width of the control impulses is varied in order to modulate the droplet velocity and the droplet volume. This is intended to create various stages of gray. A variation of the impulse width results in a change of the droplet size. The large number of impulse parameters requires expensive tabulation. Because of the complexity of the table, it is only possible to operate all channels with the same impulse voltage and the same impulse width.
In both of the previously known solutions, the print image may be affected if the printhead is moved at constant relative velocity vis-a-vis the paper to be imprinted.